1. Field of the Invention
The present invention relates to a method of and an apparatus for measuring the information hold time of memory cell in a MIS (Metal-Insulator-Semiconductor) dynamic memory.
2. Description of the Prior Art
In a MIS dynamic memory wherein information is held by means of electrostatic capacitive charges, the recharging, i.e. refresshing at certain intervals is required during the performance of the memory. Therefore, it is important to measure the hold time for stored information of memory cell.
In the prior art method, the information hold time of a memory cell at a predetermined address in a MIS dynamic memory is measured by writing an information in that address and reading the written information when a predetermined time has lapsed after the interruption of application of clock pulses. In the actual operation, however, the information hold time of an address or bit is considerably decreased by the access to the neighboring addresses. For example, it has been revealed that the information hold time after the cease of clock pulses is about 100 ms but that when the access to the neighboring addresses is carried out, the hold time is reduced to 1 to 10 ms. This means that if the refreshing cycle for the memory cells is selected on the basis of the value obtained by the conventional measuring method, an erroneous operation may easily occur in the actual performance. Moreover, if the quality of a memory is estimated according to this conventional method, misjudgment may be incurred.